[disillusioned19]

I have an experiment that I need to do, which starts off with the vanadyl [VO]²⁺ ion, which I then complex with acac. I then have to add adducts ( pyridine etc) to see the influence on the V=O stretch. Is an adduct just a ligand substitution? I'm confused what an adduct actually is, and so I can't really predict how the V=O stretch is going to change?

Furthermore, what sort of chemical effects do I need to look into to answer the question fully?

Thanks in advance.

[cheese (MSW)]

An adduct is a complex formed between two chemically stable species usually in a 1:1 ratio (but you can have 2:1 adducts as well):     
L:  +  [M]   ⇋   L:→[M]
Note this was the original definition of a complex: a chemical species formed by the combination of two stable chemical entities.  The product was more complex.
Your system is:  L:  +  V(acac)2=O   ⇋  L:→V(acac)2=O
How many isomers are there of LV(acac)2=O?  (Beware - it is a trick question!)
An old ref for you is:    J. Selbin, Chem Rev 1965, 65, 153.  It is an e-journal and the article can be read (downloaded) on-line (all 112 years of the issues: amazing).  This is a good place to start.
I go on about how Cotton & Wilkinson is the inorganic chemists’ "Bible”, but the Bible for IR (Raman) spectra of inorg cmpds is Nakamoto:
Infrared and Raman spectra of inorganic and coordination compounds 6th ed Kazuo Nakamoto.
Wiley, 2009.  Part B 1.22 Metal Complexes Containing Oxo Groups  p 175
I hope you know where the library is. 
As for how to interpret your shifts in ν(V=O) that is tricky (what level are you?).  Start with looking at the frontier orbitals on V and O and match up the overlaps (H*OMO-LUMO); then think about how these change with a change in the σ/π properties of L.  Good hunting.